Japanese authorities are investigating nearly a dozen wooden boats carrying decomposing bodies found drifting off the northwestern coast over the past month.

Coast Guard officials said Tuesday they have found at least 11 shoddy boats carrying the bodies of unknown nationality since late October. They have also found fishing equipment and nets on board and signs written in Korean, suggesting they came from North Korea.

This year, officials have found 34 mystery boats so far, including the 11 found between late October and November. Last year, Japan found 65 of them, and 80 more in 2013, according to the Coast Guard.

Coast Guard spokesman Yoshiaki Hiroto said the boats are apparently from the Korean Peninsula, though he declined to identify the country. He said the number of drifting mystery boats tends to rise during the fall and winter season because of the wind from the northwest. Simple wooden boats like those are not used in Japan, he added.

On Nov. 20, officials found four bodies in two capsized boats off the coast of Ishikawa prefecture. Two days later, seven bodies were found inside another wooden boat off nearby Fukui prefecture.

Coast Guard said officials autopsied some of them but could not determine their nationality or cause of death.